This invention relates generally to electric propulsion systems for self-propelled traction vehicles, and it relates more particularly to such a system for driving a traction vehicle which in operation is characterized by repeated reversals in direction.
In one embodiment of the invention to be disclosed hereinafter, the electric propulsion system is intended to drive earthmoving machines known technically as "wheel loaders" (standard J1057 of the Society of Automotive Engineers) and popularly as "front end loaders." A front end loader comprises a self-propelled vehicle with an integral front-mounted bucket supporting structure and linkage that loads earth and other materials into the bucket through forward motion of the vehicle and then lifts, transports, and discharges the load. Such a machine typically includes an articulated frame and a four-wheel drive. Both front and rear axles can be driven by an electrical system comprising a pair of variable speed reversible d-c motors (each having an armature and a field) which are energized by a generator coupled to a diesel engine or other suitable prime mover, and the bucket and its boom can be powered by hydraulic means including lift cylinders which derive their hydraulic pressure from the same prime mover. By appropriate manipulation of a speed-control pedal and a forward-reverse selector lever, an operator can control the electric drive system so as to determine, respectively, the machine's speed and direction of movement. The various operating modes of such a system include propulsion (motoring) or dynamic retardation (braking) in either a forward or reverse direction, with the bucket either loaded or unloaded; propelling the machine forward with the bucket down to penetrate a pile of earth ("crowding"); and lifting the bucket while the wheels are either stationary or moving forward or backward.
Propulsion systems for front end loaders should preferably have certain characteristics including: (1) smooth control of torque, (2) minimal wheel slip for improved tire life, (3) high tractive effort at low speeds to permit the loader bucket to readily penetrate the pile, termed "full crowd tractive effort," (4) relatively constant prime mover engine speed to permit rapid cycling and response of the bucket and boom assembly and to facilitate engine smoke control, (5) controllable vehicle speed, and (6) simplified control, such as, for example, to facilitate the repeated reversals in direction required during operation. Whereas the present invention will be described in connection with a propulsion system having the above characteristics and particularly adapted for front end loaders, it may be utilized in other types of electrical drives including those for other types of vehicles and those providing certain alternative characteristics.